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The magnetism of diluted and defective wide-gap semiconductors

In transition metal doped ZnO, the energy position of dopant 3d states relative to host conduction and valence bands is crucial in determining the possibilty of long range ferromagnetism. Density functional theory based estimates of the energy position of Co-3d states in Co doped ZnO differ substantially depending upon the choice of exchange-correlation functional. In this work we investigate many-body GW corrections on top of DFT+U and hybrid-DFT groundstates to provide a theoretical benchmark for the quasiparticle energies in wurtzite ZnO:Co. Both single shot G0W0 as well as partially self-consistent GW0 wherein the wavefunctions are held fixed at the DFT level but the eigenvalues in G are iterated, are considered. The predicted energy position of the minority spin Co-t2 states is 3.0-3.6 eV above the ZnO conduction band minimum which is closer to hybrid-DFT based estimates.

Left) Schematic showing the arrangement of crystal-field split Co-3d states in ZnO:Co relative to host valence (VB) and conduction (CB) bands. The occupied majority-spin 3d states are shaded deep-blue while the minority spin e and t2 levels are shown in red. Different DFT based approaches disagree on the placement of the t2 levels. (Right) Periodically repeated supercell of ZnO:Co showing a charge density iso-surface for an empty minority-spin Co-t2 state. The iso-surface is plotted for 5% of the maximum value. Large (grey) and small (red) atoms indicate Zn and O respectively.

The role of extended defect structures in stabilizing long range magnetic order in diluted magnetic semiconductor (DMS) motivate an investigation of the influence of grain boundaries (GBs) at the electronic structure of transition metal Co doped ZnO system.
The observed correlation between the presence of GBs and ferromagnetism will be investigated in this work with the help of ab-initio electronic structure calculations. Identifying qualitative as well as quantitative differences between the electronic structures of Co dopant in the bulk material and at the grain boundary is key to understanding this phenomenon. To this end we shall be employing self-interaction corrected density functional theory (DFT) methods incorporated into the SIESTA platform.